1. Field of the Invention
The present invention relates to a phase error detecting device, a waveform shaping device, and an optical disc device for reproducing a signal recorded over the limit of optical resolution, and more particularly, to a phase error detecting device, a waveform shaping device, and an optical disc device for performing phase synchronization on a reproduced signal using maximum likelihood decoding, such as Viterbi decoding.
2. Description of the Background Art
Recently, the shortest mark length in recording is nearing the limit of optical resolution because of the density of an optical disc that is becoming higher and higher. An increase in intersymbol interference and deterioration of SNR (Signal Noise Rate) are therefore becoming more noticeable. Under these circumstances, it is becoming popular to adopt PRML (Partial Response Maximum Likelihood) as a signal processing method. PRML is a technique combining partial response (PR) and maximum likelihood decoding (ML). It is a method for selecting a most likely signal sequence from a reproduced waveform based on the premise that known intersymbol interference will occur. This method is therefore known to achieve better performance than a conventional level determination method.
Meanwhile, a shift of the signal processing method from the level determination method to PRML poses a problem as to an evaluation method of a reproduced signal. Jitter, which is a reproduced signal evaluation index that has been used conventionally, is premised on the signal processing by the level evaluation method. Accordingly, there are cases where jitter is not correlated with the performance of PRML that uses a different signal processing algorithm from the one used in the level determination method. Under these circumstances, various new indices correlated with the performance of PRML have been proposed (see, for example, JP-A-2003-141823).
Also, a new index that enables a detection of displacement (edge shifting) between marks and spaces, which is crucial to the recording quality of an optical disc, is now being proposed. When PRML is adopted, this index should also be in conformity with the concept of PRML, correlated with the performance of PRML, and able to indicate a direction and an amount of edge shifting quantitatively pattern by pattern (see, for example, JP-A-2004-335079).
In addition, as an optical disc becomes further denser, intersymbol interference and deterioration of SNR will increase. In this case, it is possible to maintain the system margin by adopting high-order PRML. For example, an optical disc having a diameter of 12 cm and a recording capacity of 25 GB per layer is able to maintain the system margin by adopting PR 1221 ML. However, it is necessary to adopt PR 12221 ML for an optical disc having a recording capacity of 33.3 GB per layer. In view of the foregoing, it is anticipated that the tendency to adopt PRML at an order proportionately high to higher densities will continue.
Adopting high-order PRML suitable to intersymbol interference in response to enhancement of the recording line density can be described as a method for recognizing a reproduced waveform using a waveform pattern in a longer period in order to identify a reproduced signal from influences of intersymbol interference by increasing the identification resolution of the amplitude level of the reproduced waveform. For example, PR 12221 ML is PRML at a higher order than PR 1221 ML. Hence, by selecting suitable PRML according to a transmission path for influences of intersymbol interference caused by enhancement of the recording line density, it is possible to ensure the reproduction performance.
An optical disc recording and reproducing device has to generate a reproduction clock signal in synchronization with a reproduced signal during the reproduction and to decode the reproduced signal into a binary digital signal in synchronization with the reproduction clock signal. Generally, information about a reproduction clock signal is contained at the edge of a recording mark. The optical disc recording and reproducing device therefore generates the reproduction clock signal by detecting phase information about leading or lagging of the edge. However, in a case where a signal recorded, for example, at a recoding line density exceeding the limit of optical resolution is to be reproduced, there is a case where a reproduction clock signal cannot be generated because the optical disc recording and reproducing device fails to detect the phase information contained at the edge appropriately due to influences of intersymbol interference.